/proc/tunnel

/proc in kernel is how kernel send information to process that is the /proc file system. /proc allows easy access to information about processes, used by every bit of the kernel that can have anything interesting to report such as /meminfo/ having stats memory use statistics.

Everytime the file /proc/helloworld is read, the function procfs_read is called. Buffer and offset will be returned to the application read it. If the return value of the function isn't null, then the function is called repeatedly, if it never returns zero the function is called endlessly.

/proc/tunnel is how data between kernel and user space is shared. Acting like a pipe, having multiple 4K byte buffers - one that gets written in the kernel and consumed in user space and another that gets written from user space and consumed in the kernel. From user space, /proc/tunnel is accessed like a file. In the kernel, the access happens through the following functions:

• int tunnel_read(char* loc, int count) This function reads up to count bytes from the user to kernel buffer into where. The return value is number of characters actually read (< count if underflow).

  The first call to tunnel_read starts reading at the beginnning of the buffer. All subsequent calls begin reading at the location in the buffer where the previous call stopped reading.

• int tunnel_write(char *loc, int count) This function writes up to count bytes from the user to kernel buffer into where. The return value is the number of characters actually written (< count if underflow). The initial call to tunnel_write starts writing at the beginning of the buffer. All subsequent calls begin writing at the location in the buffer where previous call stopped writing.

• int tunnel_user() This function returns the number of used bytes in the kernel to use buffer.

• int tunnel_avail() This function returns the number of unused bytes in the kernel to user buffer.

Enabling the tunnel

To use /proc/tunnel add proc_fs.h in include/linux/ and proc_misc.c in /fs/proc respectively.

Please make sure you name the system call added kernel separately